Biology (Holt)

Figure 9shows a Punnett square that predicts
the results of a monohybrid cross between two
pea plants that are both heterozygous (Yy) for
seed color. One-fourth of the offspring would be
expected to have the genotype YY,two-fourths (or
one-half) would be expected to have the genotype
Yy,and one-fourth would be expected to have the
genotype yy. Another way to express this is to say
that the genotypic ratio is 1 YY : 2 Yy : 1 yy.
Because the Yallele is dominant over the y allele,
three-fourths of the offspring would be yellow,
and one-fourth would be green. The phenotypic
ratio is 3 yellow : 1 green.
Punnett squares allow direct and simple predic-
tions to be made about the outcomes of genetic
crosses. Although animal breeders and horticul-
turists are not always certain what characteristics
will turn up in the offspring, they can use the pre-
dictions from Punnett squares to cross individuals
that they know will be most likely to produce off-
spring with the desired phenotypes.

YY

yy

Y

Y

y

y

Yy

(Heterozygous)

Yy

Yy

Yy

(Heterozygous)

1 _

4 = yy(Homozygous recessive)

1 _

4 = YY(Homozygous dominant)

2 _

4 =Yy(Heterozygous)

Crosses That Involve

Two Traits

Suppose a horticulturist has two traits that she

wants to consider when crossing two plants. A

cross that involves two pairs of contrasting traits

is called a dihybrid cross. For example, she may

want to predict the results of a cross between two

pea plants that are heterozygous for seed shape

(Rround, rwrinkled) and seed color

(Yyellow, ygreen).

Determine possible gametes

To use a Punnett square to predict the results of

this cross, first consider how the four alleles from

either parent (RrYy) can combine to form gametes

that are either RY, Ry, rY,or ry(Figure A).

SECTION 3Studying Heredity 171

Then write these gametes on the top and left sides

of a Punnett square (Figure B).

Complete the Punnett square

On a separate sheet of paper, make a copy of the

Punnett square in Figure B, which has been par-

tially filled in with the predicted genotypes. Fill in

the remaining genotypes, then do the following:

•Listall of the possible genotypes that can

result.

• Calculatethe genotypic ratio for this cross.
  •Listall of the possible phenotypes that can
  result.


• Calculatethe phenotypic ratio for this cross.

Exploring FurtherFurther

Parent

R r Yy

(Round, yellow)

Possible gametes

Figure A Gametes

RY Ry rY ry

RRYY RRYy RrYY RrYy

RRYy

RrYy

RrYY

RrYy

RY Ry rY ry

RY

Ry

rY

ry

Yellow

Possible

gametes from

each parent

Figure B Punnett square

RrYy

RrYy

Yellow

Crossing two pea plants

that are heterozygous

for seed color (Yy) will

produce offspring in

the ratio shown in the

Punnett square.

Figure 9 Monohybrid cross:

heterozygous plants

2C 6A 6D